Following injection of a labeled nucleoside into the eye of goldfish, labeled nucleoside derivatives are axonally transported in the optic nerve to the tectum. Significantly less radioactivity is transported to the tectum when physiological activity in the optic axons has been abolished by intraocular injection of tetrodotoxin. It is proposed to analyse the effects of physiological activity on axonal transport in the optic nerve in order to determine a) which axonally transported materials are involved; b) whether the effect involves a change in axonal transport or in the synthesis or turnover of the transported materials; and c) whether the effect is the same in developing animals as in adults.